1. Field of the Invention
This invention relates to a process for the assembly of titanium aluminide intermetallic articles by diffusion brazing. The process is also of use for repairing or refacing such articles.
2. Summary of the Prior Art
A variety of intermetallic materials have been developed which have interesting properties and which have advantages for use in particular applications, such as aviation applications, over conventional metal alloys and superalloys. This class of materials includes TiAl or Ti.sub.3 Al titanium aluminides. This invention is particularly though not exclusively concerned with these two materials. These materials are typically shaped by casting or forging. For example, U.S. Pat. No. 4,294,615 describes a TiAl material of this kind. These materials may also include different constituents, such as Nb, Mo, Si, Zr, V, Ta, etc., depending upon the required application.
However, assembling the shaped articles has hitherto proved extremely difficult.
The known assembly techniques used for conventional metal alloy and superalloy materials are unsuitable for these novel intermetallic materials. In particular, assembly processes involving fusion (with or without a filler metal) such as energy beam, electron beam, or laser beam welding or TIG electric arc or plasma processes have the disadvantage of greatly modifying the metallurgical structure of the material. This causes difficulties when fragile materials like intermetallic compounds are assembled using such fusion processes. For example, titanium aluminide articles often experience cracking or crazing because of the severe heat stresses associated with the use of these conventional processes and because of the poor ductility of the materials. In other processes such as diffusion welding and friction welding, assembly is effected in the solid state. However, diffusion welding needs mating surfaces of precise geometry and very close control over cleanliness and heat cycle conditions. Friction welding requires special rheological qualities and is unsatisfactory for some assembly configurations.
Laboratory experiments have shown that since diffusion brazing produces zero thermal stressing, assembling titanium aluminide articles by diffusion brazing avoids cracking and crazing problems. The filler metals used in this case were those conventionally used in the assembly of titanium alloy articles and are, for example, Cu, TiCu, TiNi or 15% of TiCu and 15% of Ni. However, when diffusion is used with these known parameters, this process requires strict limitations to be observed if the results are to be of acceptable quality. In particular, the assembly gap is limited to 0.1-0.2 mm and is often necessary to apply pressure during assembly. Also, it is impossible to reface any surface of significant thickness.